Method of applying a transfer film to metal surfaces

ABSTRACT

A method is provided for applying a transfer film to a metal surface. The method comprises electrochemically treating the metal to form an oxide layer, on to which a transfer film is applied.

PRIORITY INFORMATION

This application is a divisional application of U.S. application Ser.No. 15/021,476 filed on Mar. 11, 2016, which claims priority toInternational Application No. PCT/US2013/067734 filed on Oct. 31, 2013.The entire contents of which are incorporated herein by reference.

BACKGROUND

Devices such as mobile phones, tablets and portable (e.g. laptop orpalm) computers are generally provided with a casing. The casingtypically provides a number of functional features, e.g. protecting thedevice from damage.

Increasingly, consumers are also interested in the aesthetic propertiesof the casing such as the look, colour, and style. In addition, devicessuch as mobile phones, tablets and portable computers are typicallydesigned for hand-held functionality, thus the consumer may alsoconsider the weight of the device and the feel of the casing by whichthey hold the device.

BRIEF DESCRIPTION OF DRAWINGS

By way of non-limiting examples, device casings and processes ofmanufacturing such casings according to the present disclosure will bedescribed with reference to the following drawings in which

FIG. 1 is a flow diagram illustrating an example of a method of applyinga transfer film to a metal surface

FIG. 2 is a flow diagram illustrating another example of a method ofapplying a transfer film to a metal surface

FIG. 3 is a sectional side view of an example of a metal oxide layer ona metal produced by the electrochemical treatment of FIG. 1 or FIG. 2

FIG. 4 is a perspective view of the metal oxide layer of FIG. 3

FIG. 5 is a sectional side view of an example of a coated metal surfaceproduced by the method of FIG. 1 or FIG. 2

FIG. 6 is a perspective view of an example of a metal casing with a partof the casing cut away

FIG. 7 is a perspective view of the casing of FIG. 6 afterelectrochemical treatment

FIG. 8 is a perspective view of the casing of FIG. 7 after transfer film

DETAILED DESCRIPTION

The present disclosure describes a method of applying a transfer film toa metal surface, for example the metal surface of a casing for a device.The method comprises the formation of a metal oxide layer on the metalsurface through an electrochemical treatment of the metal surface. Theelectrochemical treatment disclosed allows for a high degree of controlin determining various physical and visual properties of the metal oxidelayer. Furthermore, the metal oxide layer formed by the disclosed methodprovides a good adherence of the oxide to the metal.

The metal oxide layer formed by the disclosed methods is porous innature. This porosity can enhance the bonding between theelectrochemically treated surface and the transfer film compared tooxide layers formed by other methods.

Depending on the conditions of the electrochemical treatment and themetal being treated, the disclosed method can be used and may vary toform metal oxide layers of 1-300 μm in thickness and more preferably3-50 μm in thickness. In comparison, metal oxide layers formed by othertechniques are typically in the range of 0.001-0.1 μm.

FIGS. 1 and 2 illustrate examples of methods of transfer film a metalsurface.

Referring to FIG. 1, a metal surface (140) is provided (110). The metalsurface may be, for example, in the form of a casing for a device. Thecasing can be formed using conventional methods, such as stamping ormoulding, into the desired shape of the finished product. In oneexample, the casing is formed of a light metal, such as aluminium,magnesium, titanium or alloys thereof.

The metal surface (140) is electrochemically treated (120) to form ametal oxide layer (150) such as that shown in FIG. 3. Theelectrochemical treatment includes applying a potential greater than theoxide layer's dielectric breakdown potential to the metal surface in anelectrolytic solution.

The dielectric breakdown potential of a material is the voltage appliedvia an electric field that the material can withstand without breakingdown. When a material such as a metal oxide is treated with a potentialgreater than its dielectric breakdown potential, the breakdown resultsin a disruptive discharge through the metal oxide.

The dielectric breakdown potential of a material varies depending on anumber of factors, for example the composition, thickness andtemperature of the material.

An example of a suitable electrochemical process includes micro-arcoxidation (also known as plasma electrolytic oxidation). Micro-arcoxidation is an electrochemical surface treatment process for generatingoxide layers on metal surfaces.

In one example of micro-arc oxidation, a metal is immersed in a bath ofelectrolytic solution, typically a dilute alkali solution such aspotassium hydroxide. The casing is electrically connected so as tobecome one of the electrodes in the electrochemical cell, with the wallof the bath, typically formed of an inert material such as stainlesssteel, acting as the counter-electrode. A potential is applied betweenthe two electrodes, which may be continuous or pulsing, and directcurrent or alternating current.

As potentials used in micro-arc oxidation are greater than thedielectric breakdown potential of the metal oxide layer, disruptivedischarges occur and the resulting high temperature, high pressureplasma modifies the structure of the oxide layer. This results in anoxide layer that is porous (as shown in FIG. 4) and with the oxide in asubstantially crystalline form.

In addition, oxide layers formed in the above manner are conversioncoatings, converting the existing metal material into the oxide layer.This conversion of the metal provides a good adhesion of the oxide tothe metal relative to oxide layers deposited on the metal as occursusing other methods.

Properties of the oxide layer such as porosity, hardness, colour,conductivity, wear resistance, toughness, corrosion resistance,thickness and adherence to the metal layer can be varied by varying theparameters of the electrochemical treatment. Such parameters include theelectrolyte (e.g. temperature and composition), the potential (e.g.pulse or continuous, direct current or alternating current, frequency,duration and voltage) and the processing time.

In one example, the resulting colour of a titanium dioxide layer can bealtered by varying the voltage applied. In another example, organic acidcan be added to the electrolyte to allow for thicker oxide layers beingformed.

After electrochemically treating the metal surface (120), a transferfilm (160) as shown in FIG. 5, for example a polymer based transferfilm, can then be applied to the metal oxide layer (150). The porousnature of the metal oxide layer formed by the disclosed method canenhance the bonding between the transfer film and the metal oxide layer.

Examples of polymers that may be used in the transfer film include:polycarbonate (PC), polyethylene terephthalate (PET), glycol modifiedpolyethylene terephthalate (PET-G), polyvinyl chloride (PVC),polyacrylic polymer such as polymethyl methacrylate (PMMA),polyphenylene sulphide (PPS) and UV ink. The polymer based transfer filmmay contain inorganic or metallic nano-particles.

Examples of processes that can be used to apply the polymer basedtransfer film include: in-mould decoration, out-side mould decoration,in-mould film, in-mould label, release film and nano-imprintlithography.

The selection of the polymer based transfer film and its applicationprocess may depend on desired properties of the film. These propertiesmay include: visual, tactual and textural effects, as well as functionalproperties such as UV-protection, anti-fingerprinting or anti-bacterialcapability.

Referring to FIG. 2, the oxide layer may undergo a pre-film treatment(125) prior to the application of the transfer film (130).

As with the polymer based transfer film, the pre-film treatment of theoxide layer (125) can be used to alter the visual, tactual and texturalproperties of the casing. Examples of pre-film treatments include:baking, dyeing, painting, spray coating, sputter coating,electrophoretic deposition, nano-coating, chemical vapour deposition andphysical vapour deposition.

FIGS. 4 and 5 show examples of a metal surface as it undergoes a methodas shown in FIG. 1. FIG. 4 shows the metal surface (140) having a metaloxide layer (150) formed by the electrochemical treatment of the metalsurface (120). FIG. 5 shows a transfer film (160) on the metal oxidelayer (150).

FIGS. 6 to 8 provide an example of a casing (180) for a smart phone atvarious stages of the method of FIG. 1: FIG. 6 showing the casing havinga metal surface (140); FIG. 7 showing the casing of FIG. 6 afterelectrochemical treatment (120); and FIG. 8 showing a transfer film(160) on the metal oxide layer (150) of the casing of FIG. 7.

It will be appreciated that numerous variations and/or modifications maybe made to the above-described examples, without departing from thebroad general scope of the present disclosure. The present examples are,therefore, to be considered in all respects as illustrative and notrestrictive.

1. A method of applying a transfer film to a metal surface of a devicecasing, the method comprising: treating the metal surface with anelectrochemical treatment to form a metal oxide layer having adielectric breakdown potential, the electrochemical treatmentcomprising: applying a potential to the metal surface of the casing inan electrolytic solution, wherein the potential applied is greater thanthe dielectric breakdown potential of the metal oxide layer; andapplying a transfer film to the metal oxide layer.
 2. The methodaccording to claim 1, wherein applying the potential to the metalsurface of the casing includes applying the potential to the metalsurface utilizing pulsed direct current.
 3. The method according toclaim 1, wherein applying the potential to the metal surface of thecasing includes a dilute alkali composition in the electrolyticsolution.
 4. The method according to claim 1, wherein applying thepotential to the metal surface of the casing includes an organic acid inthe electrolytic solution.
 5. The method according to claim 3, whereinthe dilute alkali composition includes potassium hydroxide.
 6. Themethod according to claim 1, wherein the transfer film is apolymer-based transfer film.
 7. The method according to claim 1, whereinthe application of the transfer film includes applying a transfer filmcomprising polymer nano-particles, metallic nano-particles or acombination thereof.
 8. The method according to claim 1, wherein themetal oxide layer undergoes a pre-film treatment prior to applying thetransfer film to the metal oxide layer.
 9. The method according to claim1, wherein the transfer film comprises one of inorganic nano-particles,metallic nano-particles or a combination thereof.
 10. The methodaccording to claim 1, wherein the metal comprises aluminium, magnesiumor titanium, or alloys thereof.
 11. A method of applying a transfer filmto a metal surface of a device casing, the method comprising: treatingthe metal surface with a plasma electrolytic oxidation treatment to forma metal oxide layer having a dielectric breakdown potential, the plasmaelectrolytic oxidation treatment comprising: applying a potential to themetal surface of the casing in an electrolytic solution, wherein thepotential applied is greater than the dielectric breakdown potential ofthe metal oxide layer, and wherein the plasma electrolytic oxidationtreatment forms a porous metal oxide layer; and applying a polymertransfer film to the metal oxide layer.
 12. The method of claim 11,wherein the porous metal oxide layer is altered by varying a voltageapplied to the metal surface during the plasma electrolytic oxidationtreatment, and wherein the porous metal oxide layer is titanium dioxide.13. The method of claim 11, the method further comprising: treating themetal oxide layer with a pre-film treatment prior to applying thetransfer film to the metal oxide layer, wherein the pre-film treatmentis used to alter a tactual property of the device casing.
 14. Anelectrochemically treated casing for a device comprising: a metal layertreated with the electrochemical treatment to form a metal oxide layerthe metal oxide layer having a dielectric breakdown potential formed bymicro-arc oxidation of the metal layer, wherein the metal oxide layer isporous; and a polymer transfer film on the metal oxide layer.
 15. Thecasing of claim 14, wherein the metal layer comprises aluminium,magnesium or titanium, or alloys thereof.